The scarcity of licensed spectrum for cellular communications below 6 GHz has driven interest in unlicensed bands for LTE (Long Term Evolution) operation. In particular, the less crowded 5 GHz bands (currently used mostly for WiFi) have been proposed for LTE deployment, offering the potential for a substantial increase in LTE throughput. Overall, the design principles for LTE-U (LTE in Unlicensed spectrum) include integration with the licensed spectrum, minimal change to the existing LTE air-interface, and guaranteed co-existence with other systems using unlicensed spectrum, such as WiFi. Recently, Licensed Assisted Access (LAA) is a new technology considered in LTE Release 13 to meet the ever increasing demand for high data rate in wireless cellular networks by utilizing the carrier aggregation feature supported in LTE-A (LTE Advanced) to combine the data transmission over licensed primary carrier and unlicensed secondary component carriers. The 5 GHz band is of current interest in 3GPP (the Third Generation Partnership Project). For fair coexistence with the incumbent systems at the 5 GHz band, such as IEEE (the Institute of Electrical and Electronics Engineers) 802.11 based wireless local area networks (WLAN), Listen-Before-Talk (LBT) is considered as a mandatory feature of Release 13 LAA system.
Listen before talk (LBT) is an important feature for co-existence in the unlicensed band, wherein a transmitter listens to detect potential interference on the channel, only transmitting in the absence of interfering signals above a given threshold. Furthermore, different regions such as Europe have regulations concerning LBT for operation in unlicensed bands. WiFi devices use carrier sense multiple access with collision avoidance (CSMA/CA) as an LBT scheme, for example.